Foursquare pushed an update to its iOS app this week, which you easily could have missed. There are no radical user interface do-overs, and you might think it’s the same old app, with a few bug fixes and security patches.

Except, it isn’t.

There are two seemingly subtle updates to the way things work in Foursquare, with far-reaching consequences:

Tag your Facebook friends

This is a small one, but its sheer viral potential makes it way beyond noteworthy. By tagging your Facebook friends in your checkins on Foursquare, you’re immediately attracting them to the checkin (and Foursquare), which will be interesting to them, if they were there too. What makes this really, really viral, is the fact that those friends don’t need to be on Foursquare. You can tag someone who’s never heard of Foursquare before.